An organic electrolyte used for a lithium secondary battery or an electrochemical double layer capacitor (EDLC) has been currently widely used for a lithium ion battery or an EDLC as a liquid electrolyte in which a lithium salt such as LiPF6, LiBF4, LiN(CF3SO2)2, LiClO4, and LiCF3SO3 is dissolved in a polar non-protic organic solvent, which easily dissolves a lithium salt, and is also difficult to be electrolyzed, for example, an organic solvent such as carbonates such as ethylene carbonate and propylene carbonate, carbonic esters such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate, lactones such as γ-butyrolactone and 3-methyl-α-valerolactone, esters such as methyl formate, methyl acetate and methyl propionate, and ethers such as 1,2-dimethoxyethane, tetrahydrofuran, and dioxolane. However, even though the organic electrolyte has high ion conductivity, the organic electrolyte has problems in the process due to low stability caused by volatility, decomposition reactions at the electrode, and the like and the liquid state.
Among the solvents, particularly dimethyl carbonate or 1,2-dimethoxy ethane has such a low flash point that these solvents have problems in view of safety of a battery, such as a flash or an explosion caused by generation of heat at the time of overcharging or short-circuiting, and particularly, in recent years, development of a high capacitance and high output lithium secondary battery is urgently needed, and thus a problem of safety becomes increasingly an important problem to be solved. For this reason, it has been proposed to use phosphoric esters, esters or specific phosphoric ester compounds as a non-aqueous electrolytic solution (the official gazettes of Japanese Patent Application Laid-Open Nos. 2000-1995544 and 2001-126726), or to use a flame retardant compound such as an electrolytic solution (the official gazette of Japanese Patent Application Laid-Open No. 2005-276517) including a specific fluorinated ketone in a non-protic solvent.
Meanwhile, an ionic liquid is a normal-temperature molten salt composed of a positive ion and a negative ion, and has characteristics such as non-volatility, flame retardancy, non-inflammability, a wide potential window, and chemical stability, and thus has drawn attention as alternative electrolytes which may solve stability problems of the organic electrolyte. However, the ionic liquid is also in a liquid state at normal temperature, and thus has a disadvantage in which processability deteriorates when the ionic liquid is applied as an electrolyte.
In order to complement the disadvantage, the official gazette of Japanese Patent Application Laid-Open No. 2010-225511 suggests a gel-type electrolyte with improved durability, which is formed by internalizing the ionic liquid into porous particles. However, the solid electrolyte prepared by using only a porous metal oxide has no flexibility, and thus has a problem in that a product may easily break by external impact or when the solid electrolyte is applied to the product.